1. Field of The Invention
The invention relates to irradiated articles molded from thermoplastic molding compositions and more particularly relates to articles molded from compositions of irradiation resistant aromatic polycarbonate resins and sterilized by ionizing radiation.
2. Brief Description of The Prior Art
Synthetic polymeric resins have been used increasingly to mold articles useful in medicine and surgery. Examples of such articles include containers, packaging, instruments, prosthetics, tubing, and working components of treatment apparatus. The selection of a particular polymeric resin will depend on the physical properties required in the molded article.
One property necessary to many medical and surgical articles molded from thermoplastic polymeric resins, is their receptivity to sterilization procedures. A commonly preferred sterilization technique is exposure to ionizing radiation. Unfortunately, ionizing radiation may adversely impact some polymeric resins in ways unacceptable to some uses.
For example, polycarbonate resins have many properties which are advantageous to their use in many medical and surgical devices or articles. However, upon exposure to ionizing radiation they generally change from a normally desired transparency and clarity to a yellowed coloration. In addition, this yellow color continuously changes with increasing time after gamma ray exposure. Aesthetically, the yellowed coloration and the continuously changing color are not always acceptable.
A number of compounds have been used as additives to polycarbonate resins, to inhibit the yellowing of articles molded from polycarbonates and subjected to ionizing radiation. Representative of these additives are those described in the U.S. Pat. Nos. 4,624,972 (Nace); 4,657,949 (Nace); 4,757,104 (Nace); 4,804,692 (Lundy et al.) and 4,880,850 (Nelson et al). However, the presence of any additive in a polycarbonate resin molding composition generally has an effect on other desirable physical properties.
Another strategy for reducing the yellowing of a polycarbonate resin upon exposure to ionizing radiation is described in the U.S. Pat. No. 4,778,656 (Allen et al, 1988). The method entails blending the polymer with another polymer, which has the effect of improving ionizing radiation resistance of the polycarbonate. Examples of the additive polymers are polyester, polysulfone-carbonates and certain copolyesters. Like the previously described yellowing inhibitors, these additive polymers also affect physical properties in articles molded from blends of the mixed polymers.
The U.S. Pat. No. 4,933,424 recently issued to one of us (Rosenquist) describes a class of hydroxy-terminated polycarbonate resins, among which we have found a group of resins which exhibit improved color after sterilization with ionizing radiation.